1. Technical Field
The present application relates to a nonaqueous electrolyte secondary battery, particularly a lithium ion secondary battery, including a negative electrode active material containing a lithium-containing titanium oxide and a positive electrode active material having a layered structure.
2. Description of the Related Art
In recent years, various types of nonaqueous electrolyte secondary batteries have been developed. Typical nonaqueous electrolyte secondary batteries include lithium ion secondary batteries. While carbon materials have been mainly used conventionally as negative electrode active materials of lithium ion secondary batteries, lithium titanium composite oxide materials have been newly developed and drawing public attention. For example, a lithium ion secondary battery using Li4Ti5O12 as the negative electrode active material has already been commercialized.
Li4Ti5O12 can be used as an active material of a lithium ion secondary battery since it is a material having a spinel-type crystalline structure and is capable of repeatedly absorbing and releasing Li. Li4Ti5O12 absorbs and releases Li at a potential of about 1.5 V with respect to the standard redox potential (Li/Li+) of lithium. Therefore, where Li4Ti5O12 is used as the negative electrode active material in a lithium ion secondary battery, it is believed that a lithium ion secondary battery with a high level of safety is realized in which a lithium metal is unlikely to deposit on the negative electrode even if a reaction over-voltage occurs due to rapid charging, or the like. It is also characterized in that there is very little lattice expansion caused by charging and discharging.
On the other hand, an oxide material having a layered or spinel-type crystalline structure is commonly used as the positive electrode active material of a lithium ion secondary battery. Particularly, an oxide material having a layered crystalline structure has been drawing public attention as it is capable of realizing a high capacity. Typical examples include LiCoO2, LiNi0.81Co0.15Al0.04O2, LiNi1/3Mn1/3Co1/3O2, etc.
Therefore, lithium ion batteries have been developed in which a composite oxide having a layered structure and Li4Ti5O12 are used as the positive electrode active material and the negative electrode active material, respectively. For example, Japanese Laid-Open Patent Publication No. 2001-143702 proposes a lithium ion secondary battery in which a Lithium titanate compound represented by general formula LiaTi3-aO4 (where “a” denotes a number satisfying 0<a<3) is used for the negative electrode, and a compound represented by general formula LiCobNi1-bO2 (0≦b≦1), or LiAlcCodNia-c-dO2 (0≦c≦1, 0≦d≦1, 0≦c+d≦1), is used for the positive electrode.
In a conventional nonaqueous electrolyte second battery including a positive electrode and a negative electrode as described above, the irreversible capacity rate (retention) of the negative electrode in the initial charge and discharge is smaller than that of the positive electrode. Therefore, as the potential of the positive electrode decreases before the potential of the negative electrode increases during a discharge, thus reaching the cut-off voltage (end voltage) of the battery. The battery voltage reaching the cut-off voltage due to a decrease in the positive electrode potential is called positive electrode limitation. Conversely, the battery voltage reaching the cut-off voltage due to an increase in the negative electrode potential before the potential of the positive electrode decreases is called negative electrode limitation.